Methods for scanning an examination object with a CT system are generally known. These use for example circular scans, sequential circular scans with advance or spiral scans. Other types of scan that are not based on circular movements are also possible, for example scans with linear segments. At least one X-ray source and at least one detector opposite it are used to record absorption data of the examination object from different recording angles and the absorption data thus collected or projections are computed by way of appropriate reconstruction methods into sectional images through the examination object.
A method referred to as filtered back projection FBP is currently used as the standard method for reconstructing computed tomography images from X-ray CT data sets of a computed tomography device (CT device), in other words from the acquired projections. Following the data acquisition a step referred to as “rebinning” is performed in which the data generated by the beam widening out in the shape of fan from the source is re-ordered in such a way that it is present in a form as though the detector were struck by X-ray beams converging in a parallel manner onto the detector. The data is then transformed into the frequency domain. Filtering takes place in the frequency domain and the filtered data is then back transformed. With the aid of the thus re-sorted and filtered data a back projection is then performed onto the individual voxels within the volume of interest.
Iterative reconstruction methods have recently been developed. With such an iterative reconstruction method initial image data is first reconstructed from the projection measurement data. A convolution back projection method for example can be used for this purpose. A projector or projection operator intended to map the measurement system mathematically as efficiently as possible is then used to generate synthetic projection data from this initial image data. The difference compared with the measurement signals is then back projected with the operator adjoined to the projector and a residual image is thus reconstructed, which is used to update the initial image. The updated image data can be used in turn to generate new synthetic projection data in a subsequent iteration step with the aid of the projection operator, form the difference compared with the measurement signals again from this and calculate a new residual image which can be used again to improve the image data of the current iteration stage, etc. With such a method it is possible to reconstruct image data that features relatively good image sharpness and yet still has low image noise.
The quality of CT images increases with the number of X-ray quanta emitted by the X-ray source during measurement. However this number is limited by the structure of the X-ray source and cannot therefore be raised arbitrarily.